Wireless power transfer systems transfer electrical energy from one circuit to an adjacent circuit. Wireless power transfer systems may be used to supply power, charge batteries, transfer data and the like. One method of wireless power transfer is to utilize a variable current on a primary transmitter coil to create a varying magnetic field and a voltage, in a secondary receiver coil. The primary transmitter coil is responsive to a transmitting device that is transmitting energy. A receiving device is responsive to the secondary receiver coil that is receiving at least part of the transmitted energy. In another example of the disclosure the primary transmitter coil is coupled to a transmitter mat. The secondary receiving coil is placed adjacent to the transmitter mat and the secondary receiver coil electrically couples to the primary transmitter coil.
Power transfer is intended to occur between the transmitting device and the receiving device. Foreign objects receiving a portion of this transmitted energy decrease the system efficiency. These foreign objects may provide a path where eddy currents may be generated causing electrically induced thermal dissipation. During wireless power transfer, ohmic losses may be incurred in addition to magnetic field losses thereby increasing the difficulty in determining whether the transmitting device is communicating solely with the receiving device or the receiving device in addition to a foreign object. Variations in placement of the primary transmitter coil and the secondary receiver coil may decrease the efficiency of the magnetic field coupling, and thus the system efficiency. The foregoing may thus increase the general difficulty in determining whether a system is transferring electrical energy to energy dissipating foreign objects. Due to these and other problems and potential problems, improved detection of foreign objects using sensors would be useful and advantageous contributions to the arts.